The present invention relates to a method of shielding the electromagnetic radiation noise, which is connected with electronic apparatuses coping with EMC which becomes increasingly important along with the development of the high speed operation and the high density of electronic devices and circuits, and a circuit board employing the same. In particular, the present invention pertains to a circuit board which is capable of making the shielding function having an enclosure level unnecessary in order to realize the recycling of enclosure materials in the electronic apparatus in which the electromagnetic shielding is performed by using a plastic enclosure.
Due to the development of high operating speed, high density electronic devices and circuits, which has increased the quantity of electromagnetic radiation noise, there has been an increase in the frequency of malfunctions in electronic apparatuses. Therefore, the VCC (Voluntary Control Council for Interference by Information Technology Equipment) standards have been established so that the noise regulation is performed. However, under the present circumstances, the technology for taking measures to cope with EMC (Electromagnetic Compatibility)/EMI (electromagnetic Interference) depends on the qualitative and empirical know-how. In some cases, there may arise problems such as the excess of the contents of the specification, the excess of the period of time for the development, and the excess of the cost required for the measure.
Two kinds of noise sources are well known which radiate noise from the circuit board of an electronic apparatus: One is the differential mode radiation in which a current I is caused to flow through an electrical closed loop formed between a signal line and a ground layer, whereby the magnetic field in the position in the vicinity of the noise source (hereinafter, referred to as "the near-by magnetic field", when applicable) is generated to radiate the noise; and the other is the common mode radiation in which an electric potential Vn, which is generated due to the ground impedance and the like by the return current flowing through the ground layer, is transmitted to a cable and the like and the electric field is generated therefrom to radiate the noise.
The differential mode radiation can be represented by the model in which the radiation is generated from a small loop antenna with the magnetic field as the radiation source. The intensity of the electric field Ed in the position away from the radiation source (hereinafter, referred to as "the remote electric field" when applicable) at this time is proportional to the current flowing through the electrical closed loop formed between the signal line and the ground layer, the loop area, and the frequency squared. Therefore, in order to reduce the noise of this type, it is necessary to reduce the current value, the frequency of the current, the higher harmonic components of the current, and the area of the loop formed in the current path.
On the other hand, the common mode radiation can be represented by the model in which the radiation is generated from a short monopole antenna with the electric field as the radiation source. The intensity of the remote electric field Ec at this time is proportional to the length of the antenna (including a cable and the like) connected to the earth, the current flowing through the cable, and the frequency of that current. Therefore, in order to reduce the noise of this type, it is necessary to reduce the values of those parameters.
For example, in the case of the double side circuit board, as shown in the circuit board for coping with EMI which is disclosed in JP-A-62-295498, a shielding electrode layer is formed through an insulating layer on a board, whereby the loop area of the differential mode radiation viewed from the outside of the board is decreased to carry out the EMI measure. In addition, as apparent from one dimensional transmission line model which is shown in the embodiment, the effect of reinforcing an earth (GND) pattern (i.e., the promotion of the low impedance) is employed. Since that shielding electrode layer corresponds in function basically to a power source layer and a GND layer in the multilayer circuit board, it is considered that the transmission characteristics and the shielding characteristics equal to those of the multilayer circuit board can be provided. However, in the prior art case of a circuit board for coping with EMI, since the basic structure is at the same level as that of the multilayer circuit board, under the present circumstances, the more excellent performance can not be expected.
Many of the prior art shielding methods are, as shown in JP-A-62-295498 as described above, based on the one dimensional model and theory. Therefore, in the case of the various methods which have been adopted as the method of shielding the electromagnetic noise, it is impossible to make an accurate estimate of the phenomena which are occurring. As a result, the shielding measure also depending on the know-how, and the effects thereof can not be quantitatively estimated. Thus, in the present circumstances, we can not expect too much of the effects.